We have previously developed a radioreceptor assay to measure human growth hormone in cultured human lymphocytes. We have also determined that hGH caused a loss in receptor concentration. We have found that non-primate growth hormones and prolactins have measurable affinities for the human growth hormone receptor which is equal to that of human placental lactogen. Under preincubation conditions all hormones that bind to the growth hormone receptor cause a loss of receptors. Thus, we conclude that these hormones have significant intrinsic activity that is hGH-like. BIBLIOGRAPHIC REFERENCES: Lesniak, M.A. and Roth, J.: Regulation of receptor concentration by homologous hormone: Effect of human growth hormone on its receptor in IM-9 lymphocytes. J. Biol. Chem. 251: 3720-3729, 1976. Lesniak, M.A., Gorden, P., and Roth, J.: Reactivity of non-primate growth hormones and prolactins with human growth hormone receptors on cultured human lymphocytes. J. Clin. Endocrinol. Metab. 44: 838-849, 1977.